This guide explains the plugin API and how to write custom plugins. I suggest reading Plugins first if you have not done that already. You might also want to have a look at the List of available Plugins for some practical examples.

Note

This is a draft. If you see any errors or find that a specific part is not explained clear enough, please tell the mailing-list or file a bug report.

This plugin measures the execution time for each request and adds an appropriate X-Exec-Time header to the response. As you can see, the plugin returns a wrapper and the wrapper calls the original callback recursively. This is how decorators usually work.

The last line tells Bottle to install the plugin to the default application. This causes the plugin to be automatically applied to all routes of that application. In other words, stopwatch() is called once for each route callback and the return value is used as a replacement for the original callback.

Plugins are applied on demand, that is, as soon as a route is requested for the first time. For this to work properly in multi-threaded environments, the plugin should be thread-safe. This is not a problem most of the time, but keep it in mind.

Once all plugins are applied to a route, the wrapped callback is cached and subsequent requests are handled by the cached version directly. This means that a plugin is usually applied only once to a specific route. That cache, however, is cleared every time the list of installed plugins changes. Your plugin should be able to decorate the same route more than once.

The decorator API is quite limited, though. You don’t know anything about the route being decorated or the associated application object and have no way to efficiently store data that is shared among all routes. But fear not! Plugins are not limited to just decorator functions. Bottle accepts anything as a plugin as long as it is callable or implements an extended API. This API is described below and gives you a lot of control over the whole process.

Plugin is not a real class (you cannot import it from bottle) but an interface that plugins are expected to implement. Bottle accepts any object of any type as a plugin, as long as it conforms to the following API.

The Plugin API is still evolving. This integer attribute tells bottle which version to use. If it is missing, bottle defaults to the first version. The current version is 2. See Plugin API changes for details.

As long as apply() is not defined, the plugin itself is used as a decorator and applied directly to each route callback. The only parameter is the callback to decorate. Whatever is returned by this method replaces the original callback. If there is no need to wrap or replace a given callback, just return the unmodified callback parameter.

If defined, this method is used in favor of __call__() to decorate route callbacks. The additional route parameter is an instance of Route and provides a lot of meta-information and context for that route. See The Route Context for details.

The Plugin API is still evolving and changed with Bottle 0.10 to address certain issues with the route context dictionary. To ensure backwards compatibility with 0.9 Plugins, we added an optional Plugin.api attribute to tell bottle which API to use. The API differences are summarized here.

The Route instance passed to Plugin.apply() provides detailed informations about the associated route. The most important attributes are summarized here:

Attribute

Description

app

The application object this route is installed to.

rule

The rule string (e.g. /wiki/:page).

method

The HTTP method as a string (e.g. GET).

callback

The original callback with no plugins applied. Useful for
introspection.

name

The name of the route (if specified) or None.

plugins

A list of route-specific plugins. These are applied in addition to
application-wide plugins. (see Bottle.route()).

skiplist

A list of plugins to not apply to this route (again, see
Bottle.route()).

config

Additional keyword arguments passed to the Bottle.route()
decorator are stored in this dictionary. Used for route-specific
configuration and meta-data.

For your plugin, Route.config is probably the most important attribute. Keep in mind that this dictionary is local to the route, but shared between all plugins. It is always a good idea to add a unique prefix or, if your plugin needs a lot of configuration, store it in a separate namespace within the config dictionary. This helps to avoid naming collisions between plugins.

While some Route attributes are mutable, changes may have unwanted effects on other plugins. It is most likely a bad idea to monkey-patch a broken route instead of providing a helpful error message and let the user fix the problem.

In some rare cases, however, it might be justifiable to break this rule. After you made your changes to the Route instance, raise RouteReset as an exception. This removes the current route from the cache and causes all plugins to be re-applied. The router is not updated, however. Changes to rule or method values have no effect on the router, but only on plugins. This may change in the future, though.

Once all plugins are applied to a route, the wrapped route callback is cached to speed up subsequent requests. If the behavior of your plugin depends on configuration, and you want to be able to change that configuration at runtime, you need to read the configuration on each request. Easy enough.

For performance reasons, however, it might be worthwhile to choose a different wrapper based on current needs, work with closures, or enable or disable a plugin at runtime. Let’s take the built-in HooksPlugin as an example: If no hooks are installed, the plugin removes itself from all affected routes and has virtaully no overhead. As soon as you install the first hook, the plugin activates itself and takes effect again.

To achieve this, you need control over the callback cache: Route.reset() clears the cache for a single route and Bottle.reset() clears all caches for all routes of an application at once. On the next request, all plugins are re-applied to the route as if it were requested for the first time.

Both methods won’t affect the current request if called from within a route callback, of cause. To force a restart of the current request, raise RouteReset as an exception.

This plugin provides an sqlite3 database connection handle as an additional keyword argument to wrapped callbacks, but only if the callback expects it. If not, the route is ignored and no overhead is added. The wrapper does not affect the return value, but handles plugin-related exceptions properly. Plugin.setup() is used to inspect the application and search for conflicting plugins.

importsqlite3importinspectclassSQLitePlugin(object):''' This plugin passes an sqlite3 database handle to route callbacks that accept a `db` keyword argument. If a callback does not expect such a parameter, no connection is made. You can override the database settings on a per-route basis. '''name='sqlite'api=2def__init__(self,dbfile=':memory:',autocommit=True,dictrows=True,keyword='db'):self.dbfile=dbfileself.autocommit=autocommitself.dictrows=dictrowsself.keyword=keyworddefsetup(self,app):''' Make sure that other installed plugins don't affect the same keyword argument.'''forotherinapp.plugins:ifnotisinstance(other,SQLitePlugin):continueifother.keyword==self.keyword:raisePluginError("Found another sqlite plugin with "\
"conflicting settings (non-unique keyword).")defapply(self,callback,context):# Override global configuration with route-specific values.conf=context.config.get('sqlite')or{}dbfile=conf.get('dbfile',self.dbfile)autocommit=conf.get('autocommit',self.autocommit)dictrows=conf.get('dictrows',self.dictrows)keyword=conf.get('keyword',self.keyword)# Test if the original callback accepts a 'db' keyword.# Ignore it if it does not need a database handle.args=inspect.getargspec(context.callback)[0]ifkeywordnotinargs:returncallbackdefwrapper(*args,**kwargs):# Connect to the databasedb=sqlite3.connect(dbfile)# This enables column access by name: row['column_name']ifdictrows:db.row_factory=sqlite3.Row# Add the connection handle as a keyword argument.kwargs[keyword]=dbtry:rv=callback(*args,**kwargs)ifautocommit:db.commit()exceptsqlite3.IntegrityError,e:db.rollback()raiseHTTPError(500,"Database Error",e)finally:db.close()returnrv# Replace the route callback with the wrapped one.returnwrapper

This plugin is actually useful and very similar to the version bundled with Bottle. Not bad for less than 60 lines of code, don’t you think? Here is a usage example:

sqlite=SQLitePlugin(dbfile='/tmp/test.db')bottle.install(sqlite)@route('/show/:page')defshow(page,db):row=db.execute('SELECT * from pages where name=?',page).fetchone()ifrow:returntemplate('showpage',page=row)returnHTTPError(404,"Page not found")@route('/static/:fname#.*#')defstatic(fname):returnstatic_file(fname,root='/some/path')@route('/admin/set/:db#[a-zA-Z]+#',skip=[sqlite])defchange_dbfile(db):sqlite.dbfile='/tmp/%s.db'%dbreturn"Switched DB to %s.db"%db

The first route needs a database connection and tells the plugin to create a handle by requesting a db keyword argument. The second route does not need a database and is therefore ignored by the plugin. The third route does expect a ‘db’ keyword argument, but explicitly skips the sqlite plugin. This way the argument is not overruled by the plugin and still contains the value of the same-named url argument.